Keith Mularski doesn't look like someone with a lot of secrets. He has this aw-shucks demeanor, like an overgrown kid in a business suit.

But back in 2005, his first assignment with the cybercrime division at the FBI was to hang out on the underground sites where stolen credit cards are bought and sold. By 2006, he would be running one of the biggest underground sites on the Internet.

The first thing Mularski had to do was come up with his hacker handle. He chose Master Splyntr, after the name of an underground rat in the Teenage Mutant Ninja Turtles.

Mularkski had to create an entire backstory for Master Splyntr to get the criminals on the sites to trust him. So he contacted an anti-spam organization and got it to list his name as a Polish spam king. If you Googled his nickname, he would come up as a notorious spammer, known for buying stolen credit card information.

Master Splyntr was wheeling and dealing on the underground at a moment of particular upheaval in the credit card black market.

There were about four or five major sites where criminals bought and sold stolen credit cards, according to Kevin Poulsen, the author of Kingpin, a new book on cybercrime. Each site had about 1,500 users.

The websites battled for control of the market, much like everyday firms do in the corporate world. Some of the sites suffered hostile takeovers. Finally, two sites emerged as the dominant players: Carders Market and Dark Market.

Mularski was working undercover as an ordinary user on Dark Market when the hostile takeovers happened. At this point, he had already developed a friendship with the head of the site, a British hacker called Jlsi. One night, when Dark Market was under spam bombardment by Carder's Market, Mularski made his move:

I said: "You know my reputation as a spammer. I'm very good at setting up websites. I can hide them from law enforcement. I have my site ready. I have my servers ready." And he said: "Let's move it, bro."

And so one October night in 2006, as we were watching Saturday Night Live, Mularski moved Dark Market to his servers and the FBI took control of one of the biggest criminal sites for stolen credit card information on the Web.

But it didn't take the site down immediately. Instead, it turned Dark Market into a sting operation that resulted in multiple arrests.

Because of Master Splyntr's work, the credit card selling underground is fractured again. But that doesn't mean there are no more sites selling stolen credit card information. As long as card numbers are easy to get, thieves will find a way to trade them online. Which might be why Keith Mularski wouldn't tell me if he was still working undercover.

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